It's nothing out of the ordinary. Just use a full width for the picture frame. It's standard practice. Makes it easy for floor board repair, if, the hearth was changed out and the frame got damaged.

I think we need to see your situation to help at all. What kind of FP do you have ? Do your need a hearth at all? Where I live gas FPs do not require a hearth . So post pics of your space here in a comment DO NOT start another post.

Yes i need and want a hearth. Its a new wood burning fireplace. My biggest dilemma is how I am laying out my flooring surrrounding the frame / how it will notch into it etc. Multiple ways to do it

I am trying to plan how the floor around the frame will run into it. Can you share a picture of your corners?

closeup of corners